The present invention relates to an electron gun used for a CRT (cathode ray tube) or the like, and particularly to an electron gun in which an impregnated type cathode is provided in a heater built-in inner sleeve and the inner sleeve is connected to and supported by an outer sleeve by means of strap-like connecting members, called, tabs. In more particular, the present invention relates to an electron gun capable of leesening the diameter of the inner sleeve for reducing radiation heat propagating to grids and the like, thereby reducing power consumption in combination with prevention of inconveniences such as plane stray.
As shown in FIGS. 1 and 2, a CRT includes a tube main body T having a face panel portion F and a neck portion N, wherein an aperture grill AG and a fluorescent layer L are provided on the face panel portion F, and an electron gun E is provided on the neck portion N. The electron gun E, if it is of a Trinitron type, has prefocus lens system having three cathodes KB, KG and KR and, a first grid electrode G1 and a second grid electrode G2; a main lens system having a third grid electrode G3, a fourth grid electrode system G4AB, G4C, G4DE, and a fifth grid electrode G5; and a deflection system having a convergence deflection plate CP and a shield CS. In FIG. 2, reference character DY designates a deflection yoke.
In recent years, to improve the controllability of electron beams, the electron gun E is configured such that the cathode (hereinafter, represented by reference character K) and the first and second grid electrodes G1 and G2 are integrally assembled into a cathode structure A.
The cathode structure A is, as shown in FIGS. 3 and 4, configured such that the cathode K is fixed at an end portion, on the grid electrodes G1 and G2 side, of a heater 11 built-in inner sleeve 10; the inner sleeve 10 is inserted in an outer sleeve 20 and is fixed thereto by means of strap-like tabs (not shown); the outer sleeve 20 is mounted on a ceramic disk 22 via a holder 21; and the first grid G1 is directly provided on the ceramic disk 22 and the second grid electrode G2 is provided on the ceramic disk 22 via a spacer 23. In FIG. 3, reference numeral 30 designates a lead for making the grid electrode G1 conductive, and in FIG. 4, reference numeral 17 designates a cap for mounting the cathode.
In recent years, an impregnated type cathode K in which a porous base such as a tungsten sintered body is filled with a cathode material has been developed and has come to be used for the electron gun E shown in FIGS. 3 and 4 in place of a conventional oxide cathode.
As is known, the impregnated type cathode has an advantage that the electron emission density is higher than that for the oxide cathode, and therefore, the electron gun E using the impregnated type cathode K is advantageous in enhancing the performance of the CRT.
However, in the electron gun E using the above impregnated type cathode K it is essential to heat the impregnated type cathode K at a high temperature because the operational temperature of the impregnated type cathode K is higher than that of the oxide cathode. That is to say, the operational temperature of the oxide cathode is about 800.degree. C., while the operational temperature of the impregnated type cathode K is about 1000.degree. C.
As a result, there occur a first problem that a calorie transferred from the built-in heater 11 to the first and second electrodes G1 and G2 by way of the inner sleeve 10, outer sleeve 20 and ceramic disk 22 is large, thereby to increase the power consumption of the heater 11; a second problem that when the grid electrodes G1 and G2 are over-heated at high temperatures, Ba and BaO are evaporated and scattered from the cathode K and are deposited on the grid electrodes G1 and G2 to form emission sources ES (see FIG. 4), and thermal electrons are emitted from these emission sources ES, to increase the stray emission intensity, thereby exerting adverse effect not only on an image quality but also on the focus characteristic, resulting in a phenomenon (so-called plane stray) in which the center of the screen becomes bright after completion of the deflecting operation; and a third problem that a leakage current from the grid electrodes G1 and G2 is increased, to cause a malfunction of the circuit.
It is known that the above problems can be somewhat solved by reducing the area of the inner sleeve 10, that is, reducing the diameter of the inner sleeve 10, thereby reducing the radiation heat from the inner sleeve 10.
If the diameter of the inner sleeve 10 is reduced, however, a gap between the inner sleeve 10 and outer sleeve 20 becomes large, so that a calorie propagating by thermal transmission is increased, to make large the power consumption of the heater required to keep the temperature of the inner sleeve at about 1000.degree. C., and it is required to ensure the supporting rigidity of the inner sleeve 10 fixed to the outer sleeve 20 by enlarging the dimension such as the thickness of each tab in accordance with the gap between the inner sleeve 10 and outer sleeve 20, to increase a calorie propagating by thermal conduction.
To reduce radiation heat, it may be considered to reduce not only the diameter of the inner sleeve 10 but also the diameter of the outer sleeve 20; however, if the diameter of the outer sleeve 20 is reduced, a work of connecting the outer sleeve 20 with the holder 21 upon assembly of the outer sleeve 20 in the ceramic disk 22 is complicated, and increases to increase the number of assembling steps, thereby increasing the manufacturing cost.